1. Field of the Invention
The present invention relates to air conditioning control systems and more particularly relates to air conditioning control systems wherein zone temperatures are "set back" and maintained at a set back level during periods when the zones are unoccupied.
2. The Prior Art
Air conditioning control systems for single and multiple zone building structures have been proposed in which individually determinable zone temperatures are automatically maintained during periods of occupancy of the structure. These systems have been operated both pneumatically and electrically with the operation of heating and/or cooling equipment governed by pneumatic or electrical condition responsive signals produced by suitably constructed sensors. Pneumatic systems have generally produced pressure signals which vary in relation to sensed temperature changes while electrical systems have produced condition responsive signals having voltage, current, or frequency values which changes according to sensed temperature changes.
Control systems of the character referred to have become relatively sophisticated over the years in order to permit zone temperature conditions to be maintained as closely as possible to the desired "set point" temperatures of individual zones. One such system is disclosed by U.S. Pat. No. 3,788,386 issued 29 Jan. 1974 to Demaray. The Demaray patent discloses a sophisticated electrically operated multiple zone control system in a forced-air air conditioning system. Each of the individual zones is provided with a temperature sensor exposed to air in the zone and a temperature sensor for detecting the temperature of the air which is discharged to the zone from the heating and/or the cooling equipment in the building. Both of the sensors produce temperature responsive signals which vary according to changes in the sensed temperature of the medium to which they are exposed and the heating and/or cooling equipment of the air conditioning system is governed by a command signal which is derived from a combination of the sensor signals.
Systems of the general character described in the Demaray patent are extremely effective in maintaining closely controlled zone temperatures because the discharge sensor signals from the zone requiring the most heating, or cooling, control termination of cycling of the heating or cooling equipment, as the case may be, when the zone air temperature approaches the set point temperature. When the sensed temperature in such a zone is not close to the set point temperature, the operation of the heating or cooling equipment is initiated primarily by the signal produced by the zone air sensor.
In this type of system the discharge sensor signal has a relatively small authority compared to the authority of the signal produced by the zone air sensor so that the effect of the discharge sensor is to provide a "leading" or anticipating signal which prevents the temperature of the zone from overshooting the desired set point temperature.
In office buildings, schools and similar structures which are unoccupied during substantial periods of time during each day, it is frequently desirable to maintain the inside temperature of the buildings at levels which are beyond the "comfort" range when the structure is unoccupied. During the heating season the structures are heated by the air conditioning system and temperatures are thermostatically maintained at levels determined by the occupants during the periods of the occupancy of the building. The building temperatures are frequently automatically "set back" to lower levels when the buildings are unoccupied in order to reduce fuel or energy consumption.
In most circumstances the heating systems remain operative during the set back period so that the interior building temperatures are not reduced to excessively low levels. This is done to avoid the possibility of the inside temperature dropping below freezing and causing damage to interior pipes and equipment. Furthermore, heating up an excessively cold building to the comfort range sometimes requires a long period of time, is inefficient, and in many instances the tendency for the temperatures within the building to overshoot the comfort range is pronounced. Accordingly during temperature set back periods, the heating equipment is operated to maintain the building from 10.degree. to 20F.degree. below the temperature levels maintained when the structure is occupied.
Pneumatic and electrically operated control systems have been constructed to permit building temperatures to be set back during periods when the buildings are unoccupied. The operation of these systems is usually governed by a timer arrangement which initiates and terminates the temperature set-back period. An example of a pneumatically operated system which provides for set back temperatures is disclosed by U.S. Pat. No. 3,038,663 issued 12 June 1962 to Flores and which functions to control the set back temperature by changing the pneumatic control system supply pressure from that supplied to the control system during periods of occupancy to a set back pressure level. During the temperature set back period, the control system continues to operate in its normal fashion except that lower structure temperatures are maintained. In an electrical control system, this kind of operation is analogous to altering an appropriate voltage, current or frequency value of the electrical power supply for the control system.
An example of an electrical system employing a setback control is disclosed in U.S. Pat. No. 2,719,672 issued 4 Oct. 1955 to Jenkins and in which a thermally responsive bridge circuit controls operation of a furnace. A timer mechanism alters the bridge resistance to vary the zone temperatures produced between the unoccupied and occupied periods of the day.
The general principles of operation of the zone set back arrangements in the Jenkins and Flores patents, if applied to a control system of the character disclosed by the Demaray patent, would be effective to provide desired set back temperatures during periods when the buildings are unoccupied but would tend to produce some undesirable consequences.
In the first place such systems would function to maintain the zone temperatures rather precisely at the set back level by terminating operation of the heating equipment under at least partial control of the discharge sensor as the zone temperature approached the set back level. It is not necessary to maintain zones at a precise set back temperature level and cyclic fluctuations of zone temperatures are not undersirable since the zone is unoccupied anyway. Hence during set back periods, short cycling of the heating equipment is both unnecessary and inefficient.
If the electrical power supply voltage level for the control system were altered during the set back period, as suggested by the Flores patent, and a zone or part of the structure were occupied during a normal set back period, the entire structure would have to be maintained at the normal temperature level because selected setting back of individual, unoccupied zones could not be accomplished.
Altering individual thermostatic bridge resistances, as suggested by the Jenkins patent, would require a number of plural duplicate circuits in a multiple zone system and while enabling selective setting back of temperatures in certain zones, would substantially complicate the control system.